The overall goal of this proposal is to explore a novel photochemical method for killing antibiotic resistant pathogenic bacteria in localized models of infection. Photodynamic therapy (PDT) employs a non-toxic dye termed a photosensitizer (PS) and low intensity visible light, which in the presence of oxygen produce cytotoxic species. PDT has the advantage of dual selectivity in that the PS can be targeted to its destination cell or tissue, and in addition the illumination can be spatially directed to the lesion. PDT has previously been used to kill pathogenic microorganisms in vitro, but until now this has not been accomplished in animal models of infection. We have developed a novel method of targeting PS conjugates to both Gram (+) and Gram (-) pathogenic bacteria that can produce up to 6 logs of killing in vitro, while in vivo it increases the selectivity of the treatment for bacteria while sparing host tissue. This is based on the covalent attachment of the PS chlorin e6 to polycationic delivery vehicles such as poly-L-lysine, that increases the selective binding to bacteria and enables the PS to penetrate the cell walls of Gram (-) bacteria to gain access to sensitive intracellular sites. Multi-antibiotic resistant strains are as easily killed as wild-type strains. We have generated preliminary data using luminescent bacteria and a low-light imaging camera, that PDT will kill both Gram (-) species (Escherichia coli and Pseudomonas aeruginosa) and Gram (+) species Staphylococcus aureus) in vivo in animal models of both early and established infections. In the case of the invasive P. aeruginosa mice are cured of an otherwise fatal infection. Localized PDT may have an additional advantage in that it is also possible to inactivate secreted extracellular virulence factors that pathogenic bacteria use to establish infections and invade tissue. This project will seek to explore the determinants of PDT for localized infections. Four specific aims will focus on optimizing the treatment in different mouse models of early, acute and chronic infections, comprising excisional wounds, established soft tissue infection, chronic abscesses, burns and urinary tract infections. Since one of the advantages of PDT is its rapidity compared to traditional antibiotic therapy, we will also study the use of PDT to quickly reduce the bacterial burden in the infection, followed by antibiotics to eliminate the residual bacteria.